1. Field of the Invention
The present invention relates to a photolithography technology to fabricate semiconductor devices, more particularly, to an alternating phase shift mask (alt. PSM) capable of reducing or eliminating pattern deformation without repeated engineering efforts.
2. Description of the Related Art
Photolithography is widely used in the semiconductor industry to form a wide range of structures in integrated circuit chips. As the size of the chips decreases, optical phenomena such as diffraction and interference become increasingly important as they can adversely affect the resolution of the photolithography rendering further reduction in size and increases in density more difficult to realize. To minimize such phenomena and extend the range of photolithography, a technique known as phase shift mask (PSM), based on phase destructive interference of the waves of incident light, was developed. Phase shift mask shifts the phase of one region of incident light waves approximately 180 degree relative to an adjacent region of incident light waves to create a more sharply defined interface between the adjacent regions than is otherwise possible.
Recently, alternating phase shift masks have been adopted and investigated in patterning storage nodes of dynamic random access memory (DRAM). The alternating phase shift masks include row-type, column-type, and checkerboard type. FIG. 1 and FIG. 3 show an alternating phase shift mask with row-type according to the prior art.
As shown in FIGS. 1 and 3, the alternating phase shift mask 30 includes a transparent substrate 1 consisting of quartz materials and a chromium light-shielding layer 3 disposed on the transparent substrate 1. The light-shielding layer 3 has a transparent array consisting of a plurality of first phase (0 degree) rows I and a plurality of second phase (180 degree) rows II alternately interposed between the first phase rows I.
FIG. 2 is a top view showing the photoresist pattern transferred from the alternating phase shift mask 30 of FIG. 1. Storage node array 10, transferred by the transparent array of the alternating phase shift mask 30, is formed on the photoresist layer. The storage nodes 10a transferred by the outermost transparent rows 20, 22 (the top row and bottom row) tend to pattern deformation. This can result in worse critical dimension (CD) control at the DRAM array edge.
One method to compensate for the pattern deformation is to use a specific mask having a modification factor for the outermost rows or columns. However, the modification factor of the specific mask needs to be optimized by repeated engineering efforts such as experience and simulation.
Therefore, there remains a need for an improved alternating phase shift mask to compensate for the storage node deformation caused by the outermost transparent rows.